Internal combustion engines convert chemical energy in a fuel to mechanical energy. As part of the conversion, an engine may combust fuel and air, generating several products, or emissions. For example, when hydrocarbons are used as fuel, combustion products can include CO2 and NOx.
In an attempt to reduce tailpipe emissions, efforts have been made to utilize catalysts to react with the undesired emissions, thereby releasing alternative substances from the tailpipe. Other attempts have been made to decrease emissions by decreasing the operating time of an engine, for example, by using an electric motor to provide driving power under some driving conditions.
The inventors herein have recognized that several issues are raised by using a catalyst, both with and without an electric motor. In particular, while catalysts can be very effective at converting tailpipe emissions when an engine is operating under normal conditions, catalyst performance can be hindered if an engine pumps oxygen, or another undesired substance, to the catalyst. Such substances can be pumped to a catalyst when an engine is stopping, in particular if combustion has ceased but the pistons are still moving. This can be a particularly pertinent problem with a hybrid electric vehicle, where an engine may frequently stop and start.
At least some of the issues associated with saturating a catalyst with oxygen, or another undesired substance, may be addressed by quick-stopping an engine such that when the engine is stopping, burnt combustion gases are pumped to the catalyst instead of oxygen or other undesired substances. In this way, it may be possible to limit tailpipe emissions and/or improve catalyst performance.